This invention relates to electromagnetic couplers, and especially to such couplers using mode transformation, and their use in conjunction with antenna arrangements.
The invention arose out of consideration of the problems associated with the design of broadband antenna feed systems for use in spacecraft. Some current communication spacecraft operate in a transmit (Tx) band extending from 3.2 to 4.2 GHz and in a receive (Rx) band extending from 5.925 to 6.725 GHz. These bands may be referred to as a xe2x80x9c4/6xe2x80x9d GHz frequency band. The purpose of such couplers is to allow a single antenna to receive and transmit signals within its receive and transmit bands with isolation, and preferably high isolation, between Tx and Rx ports, or between the transmit and receive signals of two disparate frequency bands. Electromagnetic couplers for communications use tend to require a combination of broad bandwidth, low losses, isolation between Tx and Rx ports, light weight, simplicity and ruggedness. A compromise is ordinarily required among these and other limitations, such as cost.
Arrangements for frequency re-use of antennas are described beginning at page 371 and extending to page 445 of the text Waveguide Components for Antenna Feed Systems: Theory and CAD, by Uher et al., published 1993 by Artech House of Boston and London, ISBN 0-89006-582-9.
A coupler suitable for such use is described in U.S. Pat. No. 3,992,621, issued Nov. 25, 1975 in the name of Gruner. The Gruner arrangement includes an inner circular waveguide for propagating the 6 GHz signals and an outer circular waveguide for propagating the 4 GHz signals. The coupling section includes a plurality of inwardly projecting annular corrugations.
Thus, a coupler with mode transformer according to an aspect of the invention is for coupling (a) a common square waveguide port with at least one of nominally mutually independent (b) first external and (c) second external ports. The first external port is in the form of a cluster of first, second, third, and fourth clustered square waveguide ports. The second external port is in the form of first and second rectangular ridged waveguide ports, which are associated with corresponding waveguides. The first external port, in one embodiment, operates at a relatively high frequency band, namely 6 GHz, and the second external port operates at a relatively low frequency band, while the common port operates at both frequency bands. The common square waveguide associated with the common square waveguide port, and each of the four square clustered waveguides associated with the first external port, are capable of supporting either, or both, of two mutually orthogonal linear polarizations. In general, at any one time, one of the square waveguides may support a first linear polarization, a second linear polarization orthogonal to the first, or either of two hands of circular or elliptic polarization which has as components such linear polarizations. The second external port is in the form of first and second rectangular ridged waveguide ports, each of which is capable of supporting a single linear polarization, and each of which is associated with a corresponding ridged waveguide. The common port can couple signals with any of these polarizations with (or to) one or the other of the first and second nominally independent ports. The coupler includes a ridged square waveguide section defining a port coupled to, or in common with, the common square waveguide port and also defining a first internal square port. The ridged square waveguide section includes first and second mutually spatially orthogonal ridge structures lying between the first and first internal square ports of the ridged square waveguide section. These ridge structures tend to concentrate the fields of the dominant TE1,0 mode of either of the two mutually spatially orthogonal linear polarizations in, or into, a region near the axis or center of the ridged square waveguide section. The ridged square waveguide section also includes first and second planar phase shifters. The first planar phase shifter lies parallel to the plane of the first ridge structure, and the second planar phase shifter lies parallel with the plane of the second ridge structure, so that the first and second planar phase shifters are mutually orthogonal. Each of the first and second planar phase shifters is located between that one of the ridge structures with which it is parallel and a side wall of the ridged square waveguide section. The locations of the phase shifters are selected for propagating either polarization of the dominant TE1,0 mode from the first port to the second port of the ridged square waveguide section without substantially affecting the dominant TE1,0 mode, and for delaying by substantially xcfx80 a spatial portion of one of (a) a TE2,0 and (b) a TE0,2 mode propagating therein, to thereby convert between the dominant TE1,0 mode at the first port of the square waveguide section and the one of the (a) TE20 and (b) TEO0,2 mode at the second port of the square waveguide section. The coupler includes a transition section of waveguide. The transition section of waveguide defines a first internal square waveguide port which is coupled to the first, internal, square waveguide port of the ridged square waveguide section, and also defines a second square waveguide internal port. The transition section of waveguide includes a first septum extending completely across the square cross-section at the second internal port of the transition section of waveguide to thereby define first and second internal rectangular waveguide ports. The first septum progressively reduces in size (becomes smaller) toward the first square waveguide port of the transition section of waveguide. The transition section of waveguide converts between either polarization of the TE1,0 mode at the first internal square port of the ridged waveguide section and at least one of the TE2,0 and TE0,2 modes in the first and second internal rectangular ports of the transition section. The coupler further includes an eight-port waveguide branch section defining the first and second rectangular ridged waveguide ports of the second nominally independent port of the coupler. The branch section also includes first, second, third, and fourth clustered square waveguide ports of the first nominally independent port of the coupler, and third and fourth internal rectangular waveguide ports having a common or joined wall. The third and fourth internal rectangular waveguide ports of the branch section are coupled to the first and second internal rectangular waveguide ports of the transition waveguide section. The branch section further defines first and second H-plane walls parallel with the common wall, a first E-plane rectangular aperture in the first H-plane wall which is coupled to the first rectangular waveguide port of the second nominally independent port of the coupler, and a second rectangular aperture in the second H-plane wall which is coupled to the second rectangular waveguide port of the second nominally independent port of the coupler. The branch section further includes a second septum extending from that edge of the first rectangular aperture which is adjacent the first nominally independent waveguide port to the first nominally independent waveguide port, to thereby aid in defining the first and second clustered square waveguide ports, and further includes a third septum extending from that edge of the second rectangular aperture which is adjacent the first nominally independent waveguide port to the first nominally independent waveguide port, to thereby aid in defining the third and fourth clustered square waveguide ports.
In a particular avatar of this aspect of the invention, the coupler includes a first rectangular waveguide section extending from the first rectangular aperture to the first rectangular waveguide port of the second nominally independent port of the coupler. This particular avatar also includes a third ridge which extends through the first rectangular aperture on that side of the first rectangular aperture which is remote from the first nominally independent waveguide port. This third ridge extends, in contact with a wall of the first rectangular waveguide section at least part-way from the first rectangular waveguide aperture to the first rectangular waveguide port. A variant of this particular avatar has the third ridge extending, in contact with the first H-plane wall, from the first rectangular aperture toward the first rectangular internal port of the branch section.
In another avatar of this aspect of the invention, the first rectangular aperture in the first H-plane wall of the branch section of the coupler has at least one of height and width dimensions, in a direction transverse to the direction of propagation therethrough, less than the corresponding dimension of the third internal rectangular waveguide port.
The second septum of the branch section of the coupler may include a tapered portion extending generally across, but not in contact with, the first rectangular aperture in the first H-plane wall. This second septum may be in contact with the common wall of the waveguide associated with the third internal rectangular waveguide port.
The ridge structure of ridged square waveguide section of the coupler, in a region lying between the first or external port and the first internal port of the ridged square waveguide section may include first and second mutually coplanar ridge portions in electrical contact with mutually opposed walls of the ridged square waveguide section. In a is particular variant, the first ridge structure lying between the first and first internal ports of the ridged square waveguide section further comprises a third planar ridge portion (part of the central ridge structure) coplanar with the first and second mutually coplanar ridge portions, which third planar ridge portion is centered in the ridged square waveguide section and not in electrical contact with any wall of the ridged square waveguide section. This third planar ridge portion may be supported by a dielectric support structure. In one hypostasis of the invention, the planar phase shifter is in the form of a dielectric plate.